This invention relates generally to dentistry, and more particularly to dental anchors or pins used for retaining or reinforcing dental restorations.
When the crowns of teeth are severely decayed or fractured, difficulty is encountered in conventional preparations for providing effective retention for the restoration. Techniques are available for utilizing small retentive and/or reinforcing anchors or pins for facilitating such retention. Generally, the decayed or fractured portion of the tooth is removed so as to provide an excavated surface. Then, one or more blind channels are formed into the remaining tooth portion, extending into the dentin from the exposed excavated surface. Reinforcing or anchoring pins are then inserted into the channels. Numerous types of anchors are available including self-threading anchors, pins which are inserted and then retained by cementation, or force fitting anchors retained by an interference fit. Such anchors are inserted so that a free end portion protrudes above the excavated surface. A superstructure is then built upon the exposed excavated surface which is retained by means of the exposed free end portion of the anchors. This method of building up the superstructure on dentition is described in numerous patents and technical articles, by way of example in my U.S. Pat. No. 3,434,209.
The particular pins or dental anchors which are utilized to interconnect the superstructure and anchor it onto the understructure, have been of various types. In my U.S. Pat. No. 3,675,328, there is described a dental anchor having a plurality of threaded sections with each section being capable of being severed apart from the other. In my U.S. Pat. No. 3,675,329, there is provided an anchoring pin with a head, wherein the head extends above the understructure so as to be embedded in the superstructure and provide resistance to displacement. A further dental anchor is provided in my U.S. Pat. No. 4,053,982, which utilizes an L- shaped manipulating portion which can be received in a dental tool for manipulating the anchor into the understructure and also for assisting in the retention of the superstructure. Various types of manipulating portions can also be provided on the dental anchor, as shown in my U.S. Pat. No. 4,202,101, which also describes a hand held driver for use in inserting in the dental structure.
U.S. Pat. No. 3,861,043 describes a dental pin used in building a superstructure on a tooth, the pin including a shaft having a flute at the lower end thereof, with the remainder of the shaft being threaded.
U.S. Pat. No. 4,171,569 describes a dental pin having spaced apart threads formed on a pair of opposing coaxial shafts separated at their center by an enlarged portion serving as a rotating handle. The patent provides for a recommended major diameter of the pins and height of the threads; however it does not provide for any suggestions concerning the number of threads per inch or the pitch of the threads. U.S. Pat. No. 4,334,865 describes a dental pin having a threaded rod with a saw toothed type of thread and an enlarged collar portion serving as a bearing member for the screw.
U.S. Pat. No. 4,365,958 describes a combined dental drill and anchor pin having a stop member disposal therebetween, whereby the drill forms its own hole in the tooth dentin so that the anchor pin is secured in a single insertion step. However, this patent, as the above mentioned patents, does not provide any information with respect to the number of threads per inch or the pitch of the threads.
In the application of anchors of this type, it is of course essential that the practitioner avoid the generation of possible stress upon the dentition during the process of installation. Unless such precautions are observed, crazing or cracking of the understructure may result. The reduction or complete elimination of such possibility has been found to be extremely desirable. In order to eliminate this possibility, it has been found desirable to provide a means to reduce the possibility of excessive insertion stresses, wherein the anchor bottoming against the blind end of the channel and generating possible undesirable stress is eliminated.
Because of the importance of such superstructures in restoring the teeth, numerous technical studies and research programs have been carried out to determine the best type and technique of pin retention. In carrying out such studies, it was necessary to evaluate anchors by insertion into materials which can substitute for the dentin. Such models do not necessarily duplicate the various physical characteristics of the dentin, although typically, high modulus plastic material is utilized as test substitutes for the dentin.
Utilizing these techniques, investigators have studied various parameters effecting the retention and insertion techniques. Parameters which have been studied in detail include anchoring pin type, pin diameter, pin depth, number of pins, the effect of cavity varnish and various dental cements, and the retention in dental amalgam and the like.
For example, it has been found that self-threading anchors have the greatest retention of various types tested. It was also found that retention increases as the diameter of the pin is increased. Greater depth of insertion also increases the retention; however, self-threading anchors have been found to be most retentive for the same depth when compared to friction lock anchors or cemented pins.
Tests have also been carried out in connection with the number of anchors to be utilized, the angle of insertion, and the technique involved in inserting the anchors.
Despite all of the various tests and procedures which have heretofore been carried out, the use of dental anchors has still not been as effective and successful as is theoretically possible. One of the most obvious reasons for the lack of success is that the testing models do not accurately reproduce the dentin characteristic. In addition to having a wide range of moduli values and different compressive and tensile strengths, the dentin has a unique histological structure which includes directional properties. Furthermore, the characteristics of dentin vary, consequently the anchoring technique, for its insertion, must have such characteristics as to be consistently useful and preclude the possibility of injury to surrounding dentition. Accordingly, the various tests which use dentin substitutes to produce their results, while providing conclusions which may be effective for certain dentin properties, do not adequately address all the unique dentin properties and accordingly are not as effective as theoretically predicted.
The most important aspect of using the dental anchors is to ensure the proper amount of retention and at the same time reduce the amount of dentin compression and eliminate adverse effects upon the surrounding dentin. When prior art anchors, and especially the self-threading type of anchors, have been improperly inserted into the blind channels formed in the tooth structure, undesirable stresses have occurred on the existing dentin, especially at the blind end or bottom seat portion when fully inserted.